The inside of an X-ray detector and/or analyzer appliance, or at least the inside of the component in which X-rays propagate, is often evacuated to a degree at which for practical purposes it constitutes a vacuum. A window in the wall of the vacuum container, through which the X-rays should pass, must fulfill contradictory requirements. On one hand it should attenuate the soft X-rays as little as possible, in order not to interfere with the measurement. On the other hand it must be mechanically strong enough to withstand the pressure difference.
In this description we use the term “film” to mean a thin material layer of uniform thickness, and the term “membrane” to mean generally a structure that is relatively thin, i.e. has a very small overall dimension in one direction compared to its dimensions in the other, perpendicular dimensions. A membrane may consist of several materials and may have significant local variations in its thickness, and may exhibit structural topology, such as reinforcement ridges.
FIG. 1 illustrates the cross section of a membrane structure for X-ray detector and analyzer devices known from the patent publication U.S. Pat. No. 5,039,203. The solid, continuous window film 101 is made of e.g. diamond, beryllium or a plastic like polyimide, which can be easily grown or spun into desired thickness on the flat surface of a specifically prepared substrate. The substrate may be e.g. a silicon wafer. During the manufacturing process the other surface of the substrate is patterned with a photoresist, and the gaps in the pattern are etched away to leave a grid of reinforcement bars that appear in the cross-section of FIG. 1 as blocks 102. In other words, the same material that appeared as the substrate during the manufacturing also appears as a reinforcement in the completed structure. Wider continuous sections 103 of the combined substrate and reinforcement material frequently remain at the edges of the window to make it easier to attach it into an attachment frame.
Another membrane structure is known from patent publication U.S. Pat. No. 5,578,360. In a cross section drawing it resembles that of FIG. 1, even if the manufacturing method and the whole structural approach are completely different. The starting point is again a window film 101 made of plastic like polyimide. However, the reinforcement grid is not made of the substrate material of the manufacturing time, but of a photosensitive polymer that is spread on top of the window film. Those parts of the photosensitive polymer that should remain as reinforcement bars are exposed to ultraviolet radiation, which causes them to polymerize and solidify, while the gap portions can be removed. Finally the combination of the window film and the reinforcement pattern is detached from the substrate material.
Other prior art publications that consider membrane structures and radiation-permeable windows are U.S. Pat. No. 4,119,234, U.S. Pat. No. 4,061,944, U.S. Pat. No. 3,319,064, U.S. Pat. No. 3,262,002, and U.S. Pat. No. 2,241,432.
A thin polyimide film as such lets through gas molecules too easily to be used as the sole constituent of the window film. A barrier treatment of e.g. ceramic nature is often used to decrease the unwanted diffusion of gases through the window membrane. Barrier deposition may also be used to block out unwanted visible light or other interfering bandwidths of the electromagnetic spectrum. However, the barrier treatments have only a negligible effect in the structural considerations that are involved in this description, and can therefore be mainly omitted by mentioning that a person skilled in the art would know to add the barrier(s).
There are certain drawbacks in the membrane structures that follow the principle of FIG. 1. Using silicon as the combined substrate and reinforcement material results in modest tolerance of changes in temperature. The thermal expansion coefficients of the materials of the window film 101 and the silicon reinforcement grid are typically so different that the lateral force resulting from different amounts of thermal expansion easily causes the window film to be peeled off, especially if polymer window films are used that otherwise would have many advantages over diamond.
We may also consider the characteristic dimensions designated as A, B, C and D in FIG. 1 and their effect to the applicability of the window. The thickness A of the window film is typically little less or little more than one micrometer, like 0.3-0.5 micrometers for polyimide and 4 micrometers for diamond. In U.S. Pat. No. 5,039,203 the thickness of the silicon substrate, which in the completed product appears as the thickness D of the reinforcement grid, is 200 micrometers. In the polymer-reinforced structure of U.S. Pat. No. 5,578,360 the polyimide grid is about 300 micrometers thick. The width B of the reinforcement bars varies from the 40-50 micrometer scale of the polymer reinforcement to the 600 micrometer width of the silicon laths in U.S. Pat. No. 5,039,203, and the gap width C is about 150 micrometers in the polymer-reinforced structures and several millimeters in U.S. Pat. No. 5,039,203.
If the gap width C becomes smaller than the reinforcement thickness D, the collimating effect of the reinforcement grid begins to grow disturbingly large. In other words, since the gaps between adjacent reinforcement bars begin to resemble an array of tiny, mutually parallel tubes, the window has better permeability to radiation coming at a right angle than to radiation that comes at an oblique angle. This is often an undesired characteristic. Making the gap width larger would diminish the collimating effect, but this requires also increasing the thickness of the window film, which in turn increases unwanted attenuation. Additionally a larger structural module of the reinforcement mesh makes the thermal expansion problems worse.
It is possible to decrease the reinforcement grid thickness if a separate mechanical support mesh made of a mechanically strong material like tungsten is placed in stack with the window membrane so that the last-mentioned may lean against the support mesh. However, such an arrangement has the inherent drawback that the support mesh only helps against a pressure difference in one direction. Should the direction of the pressure difference change e.g. due to the window being placed incorrectly or due to a pressure fluctuation during a manufacturing or servicing step, the window will burst immediately onto that side that does not have a support mesh. Using two support meshes, one on each side, would introduce too much attenuation, especially if the meshes were not perfectly aligned, which is difficult.